Weighing the risks of human spaceflight
by Jeff Foust
|Despite the statistics, you’re still far more likely to encounter someone with a fear of flying than a fear of driving.|
How we perceive risk—and react to it—plays a critical role now in the aftermath of the Columbia accident. NASA officials, space program advocates, and some members of Congress have argued that NASA needs to get right back to flying the shuttle as soon as the fixes recommended by the Columbia Accident Investigation Board (CAIB) are implemented. That view is shared by a majority of Americans, according to a poll completed earlier this month by Zogby International for the Houston Chronicle: 68% said they thought the benefits of manned spaceflight outweighed the benefits. Some critics, however, have argued that it is too risky to fly the shuttle, at least with crews onboard. “An accident rate of one every 62.5 [sic] missions, through which 14 American astronauts have lost their lives, is simply not acceptable,” said Congressman Joe Barton in May. “If we had the same accident rate in our commercial aviation industry, thousands of people would be killed each day—which the citizens of this nation simply would not stand.”
This argument about the risks of human spaceflight carries over into the discussion about the proposed Orbital Space Plane (OSP). Barton, for example, thinks the OSP will be a safer alternative to the shuttle, even though no one yet knows exactly what the spacecraft will look like. “We need to spend the money on building an advanced orbiter or space plane, of the best and safest technology,” he said. An OSP, though, would be launched on an expendable booster—an Atlas 5 or Delta 4—that has been in service less than a year and may require hundreds of millions of dollars of additional engineering work to achieve the reliability required to carry a manned spacecraft.
|If commercial aviation had the same accident rate as the shuttle “thousands of people would be killed each day—which the citizens of this nation simply would not stand,” said Rep. Barton.|
The latest salvo in this debate came last week, when NASA published a document titled “Human-Rating Requirements and Guidelines for Space Flight Systems”. The document describes how the agency plans to certify the safety of future human spacecraft, such as the OSP. While the document outlines the procedures NASA would follow to human-rate a spacecraft, and includes some specific requirements (such as the need for abort and crew rescue systems), the document does not specify what level of risk is acceptable for future manned spacecraft. “Program management… shall establish, assess, and document the program requirements for an acceptable life cycle cumulative probability of safe crew and passenger return,” the document states. It does note that while future manned missions beyond Earth could have acceptable success probabilities of as low as 0.99, “considerably better performance, on the order of 0.9999, is expected for a reusable ETO [Earth to Orbit] design that will fly 100 or more flights.”
The question of what amount of risk is acceptable for human spaceflight—and who should make that determination—remains open. Answering those questions will have an effect on not just the future of the shuttle and the OSP, but also on future commercial suborbital and orbital spaceflight.